This invention relates to the field of digital demultiplexers and telemetry reformaters and specifically to the use of such reformaters with satellite and spacecraft ground stations. The 1980s' mark the beginning of the Shuttle era in spaceflight. The Shuttle Space Transportation System provides regularly scheduled launches from a reusable manned vehicle designed for low earth orbit. Two of the chief purposes of the Shuttle are to provide a frequent system of transporting payloads into space and to minimize the time needed to put payloads into space.
In this application, the term "payload" includes both packages which return to the earth with the Shuttle and satellites released from the Shuttle. The present invention is described with regard to satellites and spacecraft, but the inventive concept applies to payloads generally and also to certain non-Shuttle data and telemetry systems.
Satellites and spacecraft on the Shuttle either have orbits similar to the Shuttle's or have orbits or paths which require an additional boost. For satellites with orbits similar to the Shuttle's, the satellite is removed from the Shuttle at the appropriate time and placed into the proper orbit. If the satellite and the Shuttle have sufficiently different paths, for example the satellite is destined for a geosynchronous orbit or the spacecraft for an interplanetary path, then an Interim Upper Stage (IUS) is currently envisioned to provide the needed boost.
The currently-envisioned IUS, which can support up to four different satellites, has its own guidance system, data processor and communications equipment. The IUS can transmit data, either encrypted or unencrypted, from the payloads to a ground station and relay commands and data from the ground station to the payloads.
The planned use of the IUS and the Shuttle for transporting payloads has caused NASA and aerospace industry to reevaluate certain practices they have used since the early days of spaceflight, particularly the normal method of routing and processing satellite or spacecraft data during testing, launch and flight.
The most common procedure for testing a satellite or spacecraft during its integration is to design and use a specialized ground test station (GTS) which, among other things, receives and decodes the telemetry stream generated by a satellite or spacecraft to examine certain data in that stream.
FIG. 1 is a block diagram showing GTS 10 connected to a satellite or spacecraft 20 during integration. Typically, GTS 10 has a data processor which is programmed to decode the telemetry stream and to perform any analysis necessary for the satellite or spacecraft data to be examined.
Satellite or spacecraft data is seldom available during launch. When it is, it is usually part of the booster rocket telemetry stream. To examine the satellite or spacecraft data in the booster rocket telemetry stream, Ground Station (GS) 30, seen in FIG. 2, must be designed to decode the booster rocket telemetry stream directly or to communicate with the booster rocket GS 40. FIG. 2 illustrates these different data paths and facilities.
Since neither the booster rocket telemetry stream nor the data link from the booster rocket ground station is in the same format and at the same bit rate as the satellite or spacecraft telemetry stream, the satellite or spacecraft GS must be designed differently from the satellite or spacecraft GTS and new software must be written and tested to examine satellite data during launch.
Once separated from the booster, the satellite or spacecraft transmits its own telemetry stream to the earth. In the past, this telemetry stream was received by various tracking stations around the world which received the satellite's or spacecraft's telemetry stream and either transmitted that stream to the satellite or spacecraft GS or tape recorded that stream for later shipment to the satellite or spacecraft GS.
The telemetry stream received by the GS is usually not at the same bit rate or in the same data format as either the original stream or the booster rocket telemetry stream, so another software package must be written and tested for the satellite or spacecraft GS.
This need for multiple systems and multiple software packages to follow a satellite or spacecraft from integration to flight is unacceptable in the Shuttle era. Such a procedure is not only expensive, it requires a great deal of time, and a procedure requiring such a large expenditure of time and money is inconsistent with the purposes of the Shuttle.
The telemetry regenerator of this invention eliminates the requirement for the different GS and GTS software and hardware design and is compatible with the IUS telemetry streams as well as the Shuttle telemetry streams.
The IUS is capable of outputting two telemetry streams, shown in FIG. 3 as the IUS TMA stream and the IUS TMB stream. These two streams are outputted at either 64 Kbps or 16 Kbps. The IUS telemetry streams are formed by merging data streams from up to four individual satellite or spacecraft data bit streams with data from the IUS computers.
Each IUS telemetry stream is divided up into frames. FIGS. 4 and 5 show the frame format for the 64 Kbps and the 16 Kbps IUS telemetry streams, respectively.
As shown in FIG. 3, the Shuttle provides two different systems for telemetering payload data to ground. The first is an S-band link which can route two payload data streams of up to 64 Kbps directly to the ground. The ports of the S-band link can either be connected to either one of the IUS input streams, to satellites carried by the IUS, or to another payload.
The Shuttle also provides a Wideband Data Interleaver (hereafter and in FIG. 3, WBDI) for transmitting to earth telemetry stream of payload data up to 256 Kbps. The frame format for a WBDI telemetry stream is shown in FIG. 6. The WBDI data stream is formed by merging several payload data streams. When an IUS is flown in the Shuttle, both the IUS TMA and TMB streams can be inputted to the WBDI as can the telemetry streams from each of satellites or spacecraft on the IUS.
To prevent the necessity of multiple GTS and GS systems, the telemetry regenerator of this invention regenerates the satellite or spacecraft data bit streams on the ground in the same format and at the same bit rate as they are outputted by the satellite or spacecraft, regardless of the paths that the data in those streams have followed in being sent to ground. When a satellite or spacecraft is flown with an IUS on the Shuttle, there are several different ways that satellite or spacecraft data can be received by the satellite ground station:
1. From the unattached satellite or spacecraft or through the S-Band link (no regeneration of data needed).
2. As part of the IUS TMA (direct transmission through the Shuttle S-Band link).
3. As part of the IUS TMB (direct transmission through the Shuttle S-Band link).
4. As part of the IUS TMA stream which is in turn part of the WBDI stream.
5. As part of the IUS TMB stream which is in turn part of the WBDI stream.
6. As part of the WBDI stream directly.
An object of this invention is to regenerate the data streams merged into the Shuttle and/or IUS data streams such that data streams are available on the ground having the same data, at the same bit rate and in the same format as those outputted from the satellites.
Another object of the present invention is to regenerate those data streams even though the bit rates of the original data streams are not known precisely or are changing.
Yet another object of the invention is to regenerate those data bit streams without outputting any filler or "trash" data bits.
Additional objects and advantages of the present invention will be set forth in part in the description which follows and the part will be obvious from that description or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by the methods and apparatus particularly pointed out in the appended claims.